The present invention relates to a catalyst for the steam reforming of alcohols, which catalyst contains a palladium/zinc alloy and zinc oxide as catalytically active components. The catalyst is used in particular for the steam reforming of methanol to produce a hydrogen-rich gas that can be used as a fuel for vehicles powered by fuel cells.
The steam reforming of methanol in the presence of catalysts is a known process for producing hydrogen-rich gas mixtures, and is based on the following endothermic reaction:
Steam reforming of methanol:
CH3OH+H2Oxe2x86x923H2+CO2 xcex94H greater than 0xe2x80x83xe2x80x83(1)
The following secondary reactions may occur:
Reforming of methanol by methanol cleavage:
CH3OHxe2x86x92CO+2H2 xcex94H greater than 0xe2x80x83xe2x80x83(2)
and
CO conversion:
CO+H2OCO2+3H2 xcex94H less than 0xe2x80x83xe2x80x83(3)
In the steam reforming according to reaction equation (1) the steam is used in excess. The so-called xe2x80x9csteam to carbon ratioxe2x80x9d (S/C) is used to characterize the excess water that is used. Normally a value for S/C of between 1.2 and 2.0 is chosen. In the case of the reforming of methanol S/C is identical to the molar ratio of water to methanol.
For use in fuel cells gas mixtures are required that have a low carbon monoxide content with a high hydrogen content, since carbon monoxide deactivates the anode catalyst at which the oxidation of the fuel takes place. Normally amounts of carbon monoxide in the fuel of below 100 ppm, preferably less than 10 ppm, are required.
If the fuel is obtained by reforming methanol, this requirement can at the present time only be met by a subsequent purification of the reformate gas. The effort and expenditure involved are less the lower the carbon monoxide content in the reformate gas.
For use in vehicles, for reasons of space and weight reforming catalysts are required that have a very high specific hydrogen productivity and a high selectivity, the selectivity of the formation of carbon dioxide being used to characterize the selectivity of the steam reforming.
The specific hydrogen productivity PCat of the catalyst is understood within the scope of the present invention to denote the volume VH2 of hydrogen produced per unit mass MCat of the catalyst and reaction time t, wherein the catalyst mass is expressed in kilograms, the reaction time is expressed in hours, and the volume is expressed in standard cubic metres:                               P          Cat                =                                            V              H2                                                      M                Cat                            ·              t                                ⁡                      [                                          Nm                3                                                              kg                  Cat                                ·                h                                      ]                                              (        4        )            
The carbon dioxide selectivity SCO2 of the steam reforming is calculated with the aid of the partial pressures of the carbon dioxide PCO2 and carbon monoxide PCO that are formed                               S          CO2                =                                            P              CO2                                                      P                CO2                            +                              P                CO                                              ⁢                      xe2x80x83                    [          %          ]                                    (        5        )            
A high specific activity is the precondition for achieving a high space-time yield, which enables the volume of the reactor used in the steam reforming to be kept small. The space requirement for the gas purification can also be reduced by a high selectivity.
EP 0687648 A1 describes a two-stage process for carrying out the methanol reforming, in which the methanol is incompletely converted in the first stage in a heat transmission-optimized process at a high specific catalyst loading, followed by reaction in a conversion-optimized second stage at a lower specific catalyst loading that completes the methanol conversion. In the first stage the catalyst is charged as high as possible, preferably to produce more than 10 Nm3/h H2 per kilogram of catalyst. Pellet catalysts and also catalyst-coated metal sheets are proposed as catalyst forms.
Catalysts comprising the base metals copper, zinc, chromium, iron; cobalt and nickel are predominantly used for the methanol reforming. Catalysts based on CuO/ZnO, with which selectivities of more than 95% can be achieved, are particularly advantageous. Catalysts are known that consist completely of CuO and Zno and that can be obtained for example by co-precipitation from a solution of copper nitrate and zinc nitrate. After the co-precipitation the metal obtained is normally calcined in air in order to decompose and convert the precipitated compounds of the metals into the corresponding oxides. Finally the catalyst is reduced, for example, in the gaseous phase.
Alternatively so-called supported catalysts may also be used, in which a porous support or a finely divided, porous support material is impregnated with solutions of copper nitrate and zinc nitrate, and then calcined and reduced. In these cases aluminum oxide is mainly used as a support or support material, although zirconium oxide, titanium oxide, zinc oxide and zeoliths may also be used.
The finely divided catalyst materials thus obtained are as a rule processed into spherical shaped bodies, so-called pellets, or applied in the form of a coating to carrier bodies. These catalysts are hereinafter termed coated catalysts in order to distinguish them from the pellet catalysts. The processes known in the production of monolithic vehicle exhaust gas catalysts, may for example, be used to coat the carrier bodies. To this end the finely divided catalyst material is, for example, dispersed in water, optionally with the addition of suitable binders. The carrier bodies are then coating with the catalyst material by immersion in the coating dispersion. In order to fix the coating to the carrier body, the coated carrier body is dried and then calcined.
The carrier bodies for the coated catalysts serve only as a substrate for the catalytically active coatings. These carrier bodies are macroscopic bodies that must not be confused with the support material for the catalytically active components. Heat exchange metal sheeting or honeycomb bodies of ceramic materials or metal foils are suitable as carrier bodies. For example, the honeycomb bodies made of cordierite that are also used for purifying exhaust gases from combustion engines may be used for this purpose. These bodies comprise axially parallel flow channels for the reactants arranged in a narrow grid over the cross-section. The number of the flow channels per unit cross-sectional area is termed the cell density. The wall surfaces of these flow channels carry the catalyst coating. From DE 19721751 C1 and EP 0884273 a1 it is known that catalysts based on CuO/ZnO shrink by up to 40% and suffer a loss of specific activity during operation. DE 19721751 C1 solves the problem of shrinkage of catalyst layers on a metal sheet by introducing expansion gaps in the layers. According to EP 0884273 A1 the decreasing activity of a pellet packing of a Cu/ZnO catalyst on an aluminum oxide support can be at least partially reversed by periodic regeneration.
In JP 57007255 A2 (according to CA 96:145940) catalysts are described that are obtained by a two-stage impregnation of zirconium oxide-coated aluminum oxide pellets with one or two metals and/or metal oxides of copper, zinc, chromium, iron, cobalt and nickel, and with platinum or palladium. A typical catalyst contains 10 wt. % of copper oxide, 0.3 wt. % of palladium and 20 wt. % of zirconium oxide on the aluminum oxide pellets.
In addition to the catalysts based on base metals the noble metals of the platinum group, in particular platinum, palladium and rhodium on oxidic support materials such as aluminum oxide, titanium oxide and zirconium oxide, are also used for the reforming of methanol. These catalysts lead to the cleavage of methanol according to reaction equation (2) with a content of carbon monoxide in the product gas of up to 33 vol. %. Such catalysts are less suitable for the steam reforming of methanol. EP 0201070 A2, JP 60137434 A2 (according to CA 104:185977), JP 04362001 A (according to WPI 93-033201) and JP 03196839 A (according to WPI 91-298480) are examples thereof.
JP 60082137 describes a catalyst for the methanol cleavage that contains at least one of the noble metals platinum and palladium on an aluminum oxide support, the support having been coating with zinc oxide and/or chromium oxide in a preliminary treatment. For the preliminary coating the aluminum oxide support is impregnated with an aqueous solution of zinc nitrate and/or chromium nitrate and then calcined. Following this the pretreated support is impregnated with an aqueous solution of a noble metal compound, dried, calcined, and reduced under hydrogen.
It is furthermore known that catalysts that contain palladium on a zinc oxide support may also be employed for the steam reforming of methanol. In xe2x80x9cHighly selective supported Pd catalysts for steam reforming of methanolxe2x80x9d, Catal. Lett. 19 (1993) 211-216, N. Takezawa et al. investigated the dependence of the specific selectivity of various powdered catalysts of palladium on zinc oxide. The catalysts are prepared by impregnating zinc oxide with palladium nitrate Pd(NO3)2, drying, and calcining for 3 hours at 500xc2x0 C. in air. Powdered catalysts with a palladium content of 1 wt. % exhibit a high selectivity of 97% for carbon dioxide. The hydrogen productivity is however only 0.6 Nm3/ (kgxc2x7h).
In JP 05049930A powdered catalysts of palladium and zinc oxide are described that are produced by co-precipitation of palladium nitrate and zinc nitrate followed by calcination at 500xc2x0 C. The largest hydrogen productivity of 2.7 Nm3/(kgxc2x7h) at 220xc2x0 C. is obtained with a catalyst that contains 15 wt. % of palladium.
N. Takezawa et al. in xe2x80x9cSteam reforming of methanol over Pd/ZnO: Effect of the formation of PdZn alloys upon reactionxe2x80x9d, Appl. Catal. A 125, 1995, 145-157, point out that the catalytic performance of palladium/zinc oxide catalysts can be substantially improved by the formation of a PdZn alloy. In order to produce such a catalyst zinc oxide is first of all impregnated with palladium nitrate, dried, and calcined at 500xc2x0 C. in air for 3 hours. The PdZn alloy is formed by reduction of the catalyst at elevated temperatures. The investigations of Takezawa show that the alloy formation is complete only at reduction temperatures of 500xc2x0 C. The catalysts pretreated in this way have a very high selectivity, but a significantly lower activity than the known copper/zinc oxide catalysts Cu/Zno/Cr2O3 (30 wt. % Cu) and Cu/ZnO/Al2O3 (30 wt. % Cu) . A detailed investigation of the PdZn alloy formation is described by N. Takezawa in xe2x80x9cSelective PdZn alloy formation in the reduction Pd/ZnO catalystsxe2x80x9d, Bull. Chem. Soc. Jpn. 71, 1451-1455 (1998).
In xe2x80x9cSteam reforming of methanol over Ni, Co, Pd and Pt supported on ZnOxe2x80x9d, React. Kinet. Catal. Lett. Vol. 55, No. 2, 349-353 (1995), it is shown that in addition to Pd/ZnO, also Pt/ZnO has a very high selectivity for the steam reforming of methanol.
In xe2x80x9cNew catalytic functions of Pdxe2x80x94Zn, Pdxe2x80x94Ga, Pdxe2x80x94In, Ptxe2x80x94Zn, Ptxe2x80x94Ga and Ptxe2x80x94In alloys in the conversions of methanolxe2x80x9d, Catal. Lett. 54 (1998) 119-123, N. Takezawa et al. describe catalysts for the reforming of methanol based on alloys of the type Pdxe2x80x94Zn, Pdxe2x80x94Ga, Pdxe2x80x94In, Ptxe2x80x94Zn, Ptxe2x80x94Ga and Ptxe2x80x94In. Of the tested catalysts, Pd/ZnO at 220xc2x0 C. shows the greatest selectivity and activity in the steam reforming of methanol.
The known catalysts for the steam reforming of methanol based on palladium on zinc oxide exhibit a good carbon dioxide selectivity, which can be improved further by the selective formation of a palladium/zinc alloy. The specific hydrogen productivities of at most 2.7 Nm3/kgxc2x7h calculated from the disclosed data need to be improved further however.
Moreover the described catalysts of this type are without exception powdered catalysts, which are not particularly suitable for use in methanol reformers in vehicles. Although the catalyst powders can in principle be processed into shaped bodies such as for example tablets or spheres and then used in the form of a catalyst packing, the impaired accessibility of the reactants to the catalytically active centers in the interior of the shaped bodies automatically reduces the hydrogen productivity and thus the achievable space-time yield. This has correspondingly negative effects on the volume of the required reactor. The binders that may be needed for the shaping process reduce the hydrogen productivity still further. Furthermore, the vibrations and shocks caused when the vehicle is driven lead to an undesired abrasion of the shaped bodies, which blocks up the flow pathways in the packing and thereby steadily increases the pressure drop in the reactor.
The aforementioned coated catalysts could provide a remedy in these circumstances. Coated tests carried out by the inventors have shown however that Pd/ZnO catalyst powders form, on account of their basicity, a thixotropic coated dispersion that is difficult to process and leads into poorly reproducible coated results. In particular honeycomb bodies with a large number of cells can be coated only very inefficiently in this way.
The resulting coatings furthermore have an unsatisfactory adhesive strength. The addition of binders to the catalyst powder in order to obviate this defect is undesirable, since this reduces the achievable hydrogen productivity.
An object of the present invention is accordingly to provide a catalyst for the reforming of alcohols, in particular methanol, that has a high selectivity and specific hydrogen productivity. It is desirable that the catalyst has a hydrogen productivity of more than 20 Nm3/kgxc2x7h at a reactor temperature of 300xc2x0 C., with at the same time a carbon dioxide selectivity of more than 95%. In addition the catalyst should be able to be used at a reactor temperature up to 400xc2x0 C. A further essential aspect of the invention is the suitability of the catalyst for coated carrier bodies of ceramic material or metal without the addition of binders, which would reduce the specific productivity of the catalyst.
The above and other objects of the invention can be achieved by a catalyst for the steam reforming of alcohols that contains a palladium/zinc alloy and zinc oxide as catalytically active components. The catalyst is characterized in that the catalytically active components are deposited on at least one support material selected from the group consisting of aluminum oxide, aluminum silicate, titanium oxide, zirconium oxide, zeoliths and mixtures or mixed oxides thereof.
Preferably the catalyst according to the invention contains the palladium/zinc alloy in an amount of 0.5 to 10 wt. % and the zinc oxide in an amount of 1 to 50 wt. %, in each case referred to the total weight of the catalyst. The support material used for the catalyst should have a specific BET surface area (measured according to DIN 66132) of more than 5 m2/g, preferably more than 50 m2/g.
The catalyst is characterized by a high specific hydrogen productivity of more than 20 Nm3/kgcatxc2x7h at a reactor temperature of 300xc2x0 C., which has not hitherto been achieved by the catalysts known in the prior art. If aluminum oxide is used as support-material, then the catalyst even has a specific hydrogen productivity of up to 60 Nm3/kgcatxc2x7h at a temperature of 350xc2x0 C., with at the same time a carbon dioxide selectivity of more than 95%. This good value for the selectivity was unexpected, since as is known aluminum oxide promotes the formation of dimethyl ether as a by-product in the steam reforming of methanol (H. Takahashi et al; xe2x80x9cSteam Reforming of methanol over Group VIII metals supported on SiO2, Al2O and ZrO2xe2x80x9d; React. Kinet. Catal. Lett., Vol. 52, No. 2, 303-307 (1994)). In contrast to the results quoted in this literature reference, no formation of dimethyl ether was observed with the catalyst according to the present invention.
An active aluminum oxide is preferably chosen as support material. Finely divided aluminum oxides exhibiting the crystal structures of the so-called transition phases of aluminum oxide and having high specific surfaces of up to 400 m2/g are termed active aluminum oxides. Suitable active oxides include chi-, delta-, gamma-, kappa-, theta- and eta-aluminum oxide (see xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistryxe2x80x9d, fifth edition, Vol. A1, 560-562, 1985). In order to stabilize the aluminum oxide against thermal stresses, it may contain in a manner known per se a 0.5 to 10 wt. % of lanthanum oxide based on its total weight.
In a special embodiment the catalyst contains, in addition to at least one of the aforementioned support materials, also finely divided zinc oxide as support material for the catalytically active components. In this case too the catalyst preferably contains 0.5 to 10 wt. % of the palladium/zinc alloy and 1 to 50 wt. % of zinc oxide, in each case based on the total weight of the catalyst.
The catalyst may be formed into shaped bodies. Tablets, pellets, extrudates or granules are suitable as shaped bodies. The catalytically active components are in this case uniformly distributed over the cross-section of the shaped body. On account of the homogeneous distribution a large part of the catalytically active components is only insufficiently utilized on account of the poor accessibility for the reactants. Also, on account of the prolonged contact of the reactants with the catalytically active components in the interior of the shaped bodies there is an increased danger of the formation of by-products and thus of a decrease in the selectivity. It is therefore more appropriate if the support material is formed into shaped bodies and the catalytically active components, namely the PdZn alloy and zinc oxide, are present substantially in a 50 to 500 xcexcm thick surface shell on the shaped bodies. In this way the catalytically active components are better utilized and the selectivity of the catalytic conversion is improved.